Exhaust gas purification catalysts which are three-way catalysts obtained by coating noble metals such as platinum, rhodium and palladium onto porous metal oxide carriers such as γ-alumina, are used for efficient purification of noxious gas components, HC, CO, NOx and the like emitted from internal combustion engines.
In light of tighter worldwide emission control in recent years, it has become more difficult to conform to emission control of HC, CO and NOx, and particularly of HC immediately after start-up and NOx after warm-up. Moreover, among the noble metals mentioned above, the use of platinum or rhodium which are dwindling and expensive resources must be reduced from the viewpoint of resource conservation and cost reduction.
In order to prevent loss of catalyst performance due to deterioration (sintering) of noble metals or reduction of catalyst heat resistance due to alloying of different noble metals, it has been common in the prior art to utilize highly heat-resistant support materials as a measure against noble metal deterioration, but only limited improvement in exhaust gas purification performance has been achieved by restricting noble metal usage, and alternative views for catalyst specifications and noble metal configurations are being proposed.